1. Field of the Invention
This invention relates to a pneumatic tire having good all-weather running performances, and more particularly to a pneumatic tire having a low fuel consumption and improved all-weather running performances such as wet skid resistance, low rolling resistance, wear resistance and running performances on snow and ice roads.
2. Description of the Prior Art
Since styrene-butadiene copolymer rubbers have an excellent wet skid resistance on wet road surface and a good wear resistance, they have hitherto been widely used in tire tread. However, such rubbers are large in the energy loss and are apt to cause heat build-up, and also are large in the energy loss during the rolling, so that they are difficult to apply to low fuel consumption tires. Further, the hardening of the rubber at low temperature is considerable, and the skid performance on snow and ice roads are deteriorated, so that the application to tires on snow and ice roads is unfavorable.
As a method of simultaneously improving the wet skid resistance, the resistance to heat build-up and the rolling resistance, therefore, Japanese Patent laid open No. 57-51,503, No. 57-55,204 and No. 58-36,705 disclose the use of styrene-butadiene copolymer rubber (SBR) having the controlled contents of styrene and vinyl bond. This SBR exhibits an excellent wet skid resistance and a low fuel consumption, but is high in the glass transition temperature (Tg) and conspicuous in the rapid hardening at low temperature, so that the brittle fracture properties at low temperature are deteriorated, resulting in the degradation of skid performances on snow and ice roads.
On the other hand, although various pneumatic tires suitable for running on snow and ice roads have hitherto been proposed, their performances are not necessarily satisfied and consequently still desired to be improved. In this connection, various studies on the material of the tread portion have been made in order to improve the tread gripping force against snow and ice road surfaces, among which the reduction of rubber hardness at low temperature is well-known from C. S. Wilkerson; RCT, 27 225 (1954), F. S. Conant; RCT 22 863 (1949) and the like. For this purpose, as is known from literatures of W. G. Norich; "The Friction of Polymer on Ice" (Journal of the IRI, October, p192, 1972), Desumond Moore; "The Friction of Pneumatic Tires" (1975) and the like, the use of butadiene rubber, natural rubber and polyisoprene rubber as tread rubber improves the tread gripping force against snow and ice road surfaces. In such a method, however, the tread gripping force against wet road surface is unfavorably reduced. Furthermore, in order to solve this reduction of the tread gripping force, as is known from literatures of A. C. Bassi; RCT 38 1965, D. Bulgin, G. D. Hubberd, M. H. Walters; Proc. 4th Rubber Tech. Conf. London, 193 (1962) and the like, there is a method of using a large amount of styrene-butadiene copolymer rubber, butyl rubber or carbon black. In this method, however, the performances on snow and ice roads and the rolling resistance are unfavorably deteriorated.
In Japanese Patent Application Publication No. 57-59,256 is disclosed that the rolling resistance and the wet skid resistance of the tire can be improved without substantially sacrificing the wear resistance by adding more than 5 but not more than 30 parts by weight of a polyisobutylene-isoprene rubber (halogenated butyl rubber) containing 1.0-2.0% by weight of chlorine (Cl) or bromine (Br), or the mixture thereof to natural rubber (NR) and polybutadiene rubber (BR). However, such a rubber composition is practically insufficient in view of the improvement of the running performances on snow and ice roads aiming at the invention. Further, Japanese Patent Application Publication No. 58-46,257 discloses that a tread rubber composition obtained by adding to a blend of natural rubber (NR), styrene-butadiene copolymer rubber (SBR) containing styrene of not more than 20% by weight and polybutadiene rubber (BR) an extender oil having a flowing point of not more than -10.degree. C. in an amount of not less than 20% by weight of total extender oil content is used in a tire for truck and bus having the ice skid resistance, the wet skid resistance and the wear resistance. Although the use of such tread rubber composition improves the ice skid resistance, the wet skid resistance and the wear resitance in the tire for truck and bus to a certain extent, it is considered to be still insufficient from the viewpoint of the levels of low fuel consumption, wet skid resistance and running performances on snow and ice roads required mainly in tires for passenger cars aiming at the invention. On the other hand, in Japanese Patent Application No. 58-79,406 and No. 58-79,407, the inventors have previously proposed a pneumatic tire using in tread a rubber composition containing an ester type low-temperature plasticizer for the purpose of improving the tread gripping force against snow and ice road surfaces without deteriorating the wet skid resistance, the wear resistance and the rolling resistance. Further, the inventors have made various studies with respect to the development of tires having the dramatically improved properties or so called all-weather running performances and confirmed the followings. That is, it is desirable to improve the tread gripping force against snow and ice roads without degrading the properties required for the existing summer tires, such as wet skid resistance, wear resistance, rolling resistance and the like. However, even in the tires having low fuel consumption and so-called all-weather running performances such as wet skid resistance, rolling resistance and running performances on snow and ice roads, the levels of the tire performances required by a user are naturally different in accordance with weather conditions in areas of actually using the tire, for example, an area where snow and ice road surfaces are formed for several days in winter season, a largely rainy area, an area where the running on snow and ice roads is preferential in winter season, and the like. As mentioned above, although there are various summer tires attached importance to the rolling resistance, the et skid resistance or the like depending upon the levels required by the user, if not only the performance at the above each level is kept but also the running performances on snow and ice roads are improved, it can be said to be a tire with all-weather running performances satisfying the requirements of each user.